Head-Up Displays: Designing the Way Ahead

Richard L. Newman

• Editore: Routledge
• Anno: 2017
• ISBN:
  9781351931519
• Edizione: 1
• Argomento: Technologie et ingénierie
• Categoria: Aviation

This is a thorough description of this increasingly important technology, starting from the development of head-up displays (HUDs), particularly specifications and standards and operational problems associated with HUD use. HUD involvement in spatial disorientation and its use in recognizing and recovering from unusual attitudes is discussed. The book summarizes the design criteria including hardware, software, interface and display criteria. It goes on to outline flight tasks to be used for evaluating HUDs and discusses the impact of HUDs on flight training. Recent work indicates that a HUD may allow a significant reduction in the time required to train a pilot on a particular aircraft, even considering non-HUD-related tasks. The author concludes with a review of unresolved HUD issues and recommendations for further research and provides an impressive bibliography, glossary and index. Within the military aviation sector the book will be of use to industry, research agencies, test pilot schools and air force training establishments. In the civil area regulatory authorities, airlines and industry will also have an increasing interest.

1 Introduction

Head-up displays (HUDs) provide the pilot with a means to view real-world cues simultaneously with on-board flight information. This combination of real-world cues and artificial cues requires displays collimated at infinity.

The major advantages of head-up displays are seemingly obvious:

• Reduced pilot workload Pilot workload is reduced when the overall piloting tasks require head-up, outside-the-cockpit flight references.
• Increased flight precision The expanded scale of the HUD data and its overlay on the external visual scene allows the pilot to fly more precisely.
• Direct visualization of trajectory A conformal display allows the pilot to assess the aircraft performance directly.
• Increased flight safety Essential flight information presented on the HUD reduces eyes-in-the-cockpit during critical flight maneuvers.

Purpose

In spite of these advantages, the use of HUDs as flight instruments has not been totally successful. Since the late 1970s, a number of reports (1-4)¹ have been published citing significant deficiencies in HUD symbology and installations. These deficiencies include lack of failure detection, lack of standardization, and increased tendency toward spatial disorientation.

This document reviews historical HUD experience and recommends a set of HUD design criteria. These are based on operational experience and on HUD research where appropriate. The material should help engineers develop new head-up displays. It should also help airlines and other potential HUD operators understand the limitations of existing HUD specifications and avoid repeating mistakes of the past.

Scope

We will restrict the discussion to displays which display flight information in virtual images in the pilot's forward field-of-view. The discussion will not consider helmet-mounted displays worn by the pilot.

HUDs considered include those displays used for routine flying maneuvers, for all-weather instrument landing, for weapons delivery, and for other specialized uses. The recommendations for HUD design are intended for all fixed-wing aircraft, tactical, transport, and specialized mission aircraft. Where appropriate, recommended values will be shown for each type of aircraft.

The recommendations are based on characteristics of HUDs found in the past to have desirable characteristics based on pilot opinion (and on characteristics which have demonstrated problems in operational use).

Display design

Reviews of operational HUDs lead to the conclusion that the HUD design process itself may be deficient. There seems to be a tendency in developing HUDs to include everything that can possibly be useful to the pilot. This results in a very cluttered display.

Traditionally, display designers have sought expert pilot opinion for guidance during the development of new flight displays. While user opinion is helpful, pilots tend to have diverse (and strongly held) opinions. In addition, pilots with limited background in display evaluation often limit the design of novel systems to those concepts with which they are familiar.

Pilots also have a tendency to 'gold plate' specifications and may not realize the consequences of minor additions.

The design process

The display design must consider why the pilot needs the data and what the pilot is expected to do with the data. According to Singleton (5), the following questions should be considered during the development of a display:

• Does the pilot's need justify the display?
• What data does the pilot need that has not been provided?
• Can the average pilot obtain what is required easily?
• Does the display conform
  • - to the real world?
  • - to other cockpit displays?
  • - with previous pilot habits and skills?
  • - with required decisions and actions?

Wickens (6) proposed a 'proximity compatibility' principle to develop displays. This principle uses proximate displays for tasks that are closely related and vice versa. Others have amplified this approach for cockpit displays (7). These display design techniques have been applied to cockpit instrument panels, but not to HUDs.

Previc(8) proposed HUD design based on physiological principles. Previc's analysis appears to provide a theoretical basis for the basic instrument 'T', albeit several decades after its development.

Following completion of the display design, its evaluation must be based on objective, performance-based criteria and measures of the display's effect on mission performance. It is up to the evaluation team to determine what the appropriate measures are. These should reflect the intended mission of the aircraft and include all mission segments.

Clutter

When deciding what information to display, the designer must use the Hippocratic approach used by physicians in treating an injury or illness: First, do no harm. Every information element shown on a HUD must serve a purpose and lead to improved performance. Every pixel must be justified. While all displays have a need to minimize display clutter, this is particularly critical with see-through displays. Since HUD symbols are presented in the pilot's view of the real world, obtrusive symbology should be kept to an absolute minimum. Hughes (9) expressed this as 'not one "pixel" should be lit unless it "buys" its way onto the screen by providing a demonstrable improvement in performance.'

Global priorities vs. minutia

A final comment: display specifications and, as a result, display designers in the past have over concerned themselves with details of the specific symbols. The general arrangement (location within the field of view) and the algorithms driving the symbols are more important (in our opinion) than the details of the symbols themselves.

The evaluation process

The history of HUDs shows that poorly designed displays were not 'flagged' during the test and evaluation phase, and the deficiencies were not corrected. Because of this, some comments on the testing of displays are in order.

In the past, non-weapon system HUD evaluations were conducted fairly superficially using benign tasks with no particular criteria other than subjective opinion. While subjective opinion should not be ignored, the performance achievable with the display must be specified. The flight tasks must be rigorous, requiring high workload on the part of the evaluation pilot — even to the point of adding secondary tasks to add stress and distractions.

¹Italic numbers in parentheses, (), indicate references listed at the end of each chapter.

References

(1) Barnette,J. F. Role of Head-Up Display in Instrument Flight (Randolph AFB, Texas: Air Force Instrument Flight Center, 1976), AFIFC LR-76-2

(2) Newman, R. L. Operational Problems Associated with Head-Up Displays During Instrument Flight (Wright-Patterson AFB, Ohio: Air Force Aeromedical Research Laboratory, 1980), AFAMRL TR-80-116

(3) Lovering, P. B. and Andes, W. S. Head-Up Display Symbology and Mechanization Study (Wright-Patterson AFB, Ohio: Aeronautical Systems Division, 1984), ASD TR-84-5023

(4) Newman, R. L. and Foxworth, T. G. A Review of Head-Up Display Specifications (Wright-Patterson AFB, Ohio: Aeronautical Systems Division, 1984), ASD TR-84-5024

(5) Singleton, W. T., 'Display Design: Principles and Procedures,' Ergonomics, 12, 1969, 519-531

(6) Wickens, C. D. The Proximity Compatibility Principle: Its Psychological Foundation and Its Relevance to Display Design. (Champaign, Illinois: University of Illinois, 1992), ARL-92-5/NASA-92-3

(7) Andre, A. D., 'Quantitative layout analysis for cockpit display systems,' Proceedings of the Society for Information Display 1992 International Symposium, Boston, (New York: Society for Information Display, 1992), Paper 34.2

(8) Previc, F. H. Towards a Physiologically Based HVD Symbology (Brooks AFB, Texas: Air Force School of Aerospace Medicine, 1988), AFSAM TR-88-23

(9) Hughes, R. E. The HVD Coloring Book: Recommendations Concerning Head-Up Displays (Washington,: Naval Air Systems Command, 1991)

2 Historical review

The first head-up displays (HUDs) were developed during the late 1950s in several countries, based on reflecting gunsight technology. In these gunsights, the aiming symbol is generated from a light source and projected onto a semi-transparent mirror mounted between the pilot and the windshield. The projector is usually located in the top of the instrument panel. The aiming symbol appears to be 'floating' in the pilot's view of the outside world.

Reflecting gunsights were first used in World War II fighters and had, by the late 1950s, progressed to display images generated on cathode ray tubes (CRTs) which were controlled by airborne computers. Reflecting gunsights have several advantages over their precursors, immovable iron sights.

First, the aiming symbol can be moved to compensate for range, bullet drop, acceleration factors, and rate of target closure. With the incorporation of airborne computers, the equations of symbol motion had become quite complex.

Second, the image of t...